Abbreviations:
    3GPP 3rd generation partnership project    LTE long term evolution    LTE-A LTE-advanced    Rel Release    PDCCH Physical Downlink Control Channel    PHICH Physical HARQ Indicator Channel    PCFICH Physical Control Format Indicator Channel    CoMP Coordinated Multi-Point Transmission    CRS Common Reference Symbols    CSI-RS Channel State Information Reference Symbols    DCI Downlink Control Information    DM RS Demodulation Reference Symbols    DRS Dedicated (UE-Specific) Reference Symbols    eNB, eNodeB Base Station (evolved NodeB)    PDSCH Physical Downlink Shared Channel    RRC Radio Resource Control    TM Transmission Mode    TX Transmit(ter)    UE User Equipment    MIB Master Information Block    SIB System Information Block    PRACH Physical Radio Access Channel    PHICH Physical HARQ Indication Channel    PBCH Physical Broadcast Channel    BD Blind Decoding    SS Search Space    SFN Single Frequency Network    RRM Radio Resource Management    RACH Random Access Channel    RSRP Reference symbol received power    RSRQ Reference symbol received quality    PSS Primary synchronization signal    SSS Secondary synchronization signal    BCH Broadcast Channel    id identifier    RNTI Radio Network Temporary Identifier    C-RNTI Cell RNTI    SPS-RNTI Semi-Persistent Scheduling RNTI    CRC Cyclic Redundancy Check    MIMO Multiple Input multiple output    MU-MIMO Multi-user MIMO
LTE downlink physical layer have over the past releases been evolving from being 100% based on common reference signals (CRS) towards using more UE specific reference signals such as DM RS.
The common reference signal in LTE downlink has two main functions: 1) support UE demodulation of data and control messages, 2) support UE measurements used in RRM functions such as MIMO and Handover. It is now a clear trend to gradually reduce the usage of CRS to remove it totally in the future. 3GPP have already considered a number of new functions which can substitute partly the CRS.
By introducing DM RS and CSI-RS, PDSCH transmission doesn't rely on CRS for demodulation and channel measurement any more.
UE Measurements
For the UE measurement function, 3GPP have already in Rel-10 introduced channel state information reference signals (CSI-RS) which allows the UE to do measurements with only limited impact to the system control overhead. Measurement accuracy and reference signal density is clearly related but with current CSI-RS a number of different options are available, so RS density can be adjusted to the radio environment, such that in challenging radio environments higher density is provided.
At Rel-10, it is not possible to use CSI-RS for handover function and RSRP/RSRQ measurement; however there does not seem to be a technical problem which hinders this to be specified at a later stage. In this way all UE measurements may be done based on CSI-RS.
Data Demodulation
For data demodulation DRS (Rel-8) and DM-RS (Rel-9 and -10) was introduced. At the current stage all basic transmissions schemes can be supported with DM RS and only due to legacy UEs, CRS is still needed for data demodulation.
Scrambling and Decoding—PDCCH
For demodulation of downlink control channels such as PDCCH, the Rel-10 relay specification provides a solution (Relay-PDCCH: R-PDCCH) where the PDCCH is demodulated based on DM RS.
For scrambling of PDCCH, Rel-8 has defined that CRC parity bits of PDCCH can be scrambled by RNTI information bits. The entire PDCCH payload is used to calculate the CRC parity bits. Different types of PDCCH are associated with different RNTI. When UE decodes the PDCCH, corresponding RNTI is used to descramble PDCCH.
The control channels PHICH and PCFICH are not strictly needed if CRS is disabled. PHICH functions can already today be handled by PDCCH and as PDCCH would be embedded in the PDSCH region (like R-PDCCH) then there is no need to signal dynamically the control region size, so there is no need for PCFICH.
DMRS Sequence ID Initialization
In LTE R8, the sequence ID of DMRS port 5 shall be initialized with cinit=(└ns/2┘+1)·(2NIDcell+1)·216+nRNTI at the start of each subframe, where nRNTI is corresponding to PDCCH RNTI in PDSCH transmission and ns is slot number within a radio frame. NIDCell is physical cell identity. For DMRS port 5, NIDCell is cell-specific parameter, nRNTI is UE-specific parameter.
In LTE R9/R10, the sequence ID of DMRS port 7-14 shall be initialised with cinit=(└ns/2┘+1)·(2NIDcell+1)·216+nSCID at the start of each subframe, where for DMRS antenna ports 7 and 8, nSCID is 1 bit information, given by the scrambling identity field according to the most recent PDCCH DCI format 2B or 2C associated with the PDSCH transmission. nSCID shall be utilized to distinguish different users in MU-MIMO transmission. If there is no DCI format 2B or 2C associated with the PDSCH transmission on DMRS antenna ports 7 or 8, the UE shall assume that nSCID is zero. For DMRS antenna ports 9 to 14, the UE shall assume that nSCID is zero. NIDCell is physical cell identity.
Random Access Procedure
When UE wants to access the cellular network, a certain preamble code is transmitted. In the coming predefined subframe, UE will try to decode its random access response. RA-RNTI is used to scramble the PDCCH of this response. RA-RNTI is determined by the transmitted PRACH resource. Hereinafter, the first response is denoted as message 2. Message 2 will carry a temporary RNTI for next message decoding. After message 2 reception, UE will transmit a response message to the eNB for RRC connect request. After eNB reception of this message, eNB will send a message 4 as the confirmation to complete random access procedure. UE decodes the PDCCH of message 4 using temp RNTI descrambling. Herein message 2 and message 4 both are carried by PDSCH. In Rel 8/Rel 9/Rel 10, the demodulation of PDCCH and PDSCH is based on CRS.
Paging Message Demodulation
Paging message is initiated by network to recall a certain user. PDCCH of paging message is scrambled by Paging-RNTI (P-RNTI). P-RNTI is configured by eNB with RRC message before paging. UE will wait and blindly decode its paging PDCCH in predetermined subframe. If decoding is successful, the paging message will be captured by the UE.
DM RS and PDCCH Chicken Egg Problem
One problem already discussed in prior art is how to insure that the UE knows all relevant information needed for demodulating PDCCH. In the Rel-9 DM RS solution some parameters for DM RS initialization are signaled in PDCCH. Thus, they would not be available for demodulating PDCCH. This “chicken-egg” problem was solved for R-PDCCH in a simple way by just stating in the specification static values for these parameters. The drawback of this is that flexibility is very limited, which might be OK for the particular scenario of relay backhaul but for general situation may be not good enough.
SSS/PSS
The synchronization signals are in principle independent of CRS and need no change to operate in CRS less mode.